In the practice of nuclear medicine, certain radioactive isotopes are formulated in a nuclear pharmacy, or in a nuclear medicine “hot lab”, where they are subdivided into units from a bulk source or combined with other materials to produce diagnostic and/or therapeutic agents referred to as radio-pharmaceuticals. These radio-pharmaceuticals are important products in health-care and in commerce, whose value—in terms of their diagnostic or pharmacologic effect as well as price—is generally proportional to the amount of radioactive material present in a given sample.
A unit-quantity of a radio-pharmaceutical is typically referred to as a dose. An essential step in the preparation of each dose is an accurate measurement—or calibration—of the amount of radioactivity which is present. The amount of radioactivity contained in the dose must be known to within a prescribed degree of precision; if the dose is too small, it will not produce the required diagnostic or therapeutic efficacy, and if the dose is too large, there may be undesired or dangerous side effects due to excess exposure to radiation. Whether the dose is prepared in the nuclear medicine lab or by a commercial nuclear pharmacy, its calibrated activity must also be verified by the physician before patient administration.
A typical dose calibrator 9 of the prior art is shown schematically in FIG. 1. This particular apparatus, familiar to those schooled in the art, is called an Ionization Chamber. The ionization chamber is typically configured as two thin-walled, cylindrical metallic shells 1, 2 arranged coaxially (shown in cut-away), and separated by an air-space 3. The two shells are electrically insulated from each other and from external contact. A high-voltage power supply 6 in series with a sensitive current-measuring meter or circuit 7 are connected to the inner and outer shells. A small vial or syringe 4 containing a dose to be measured is placed near the center of the chamber. The vial or syringe is an intense source of radiation emitting penetrating, high-energy photons at a rate which is proportional to the amount of activity present—expressed in Becqerels. One Becqerel equals 1 radioactive decay per second. An older, but still widely used, unit of radioactivity is the Curie which is defined as 3.7×1010 radioactive decays per second.
The energetic photons interact with the air in the annular space 3, and also in the chamber walls 1, 2 liberating secondary electrons which also interact with air in the annular space. These interactions ionize the air, generating a small electrical current, which is measured by the current-measuring meter or circuit 7. This current is generally proportional to the amount of radioactivity present in the sample vial or syringe.
As a practical matter, the overall dimensions of the ion chamber are generally rather large compared with the dimensions of the vial or syringe in order to insure uniformity of response, i.e., independent of small deviations from ideal symmetry of placement of the sample, and to provide sufficient ion current, which is generally proportional to the intensity of radiation, but also proportional to the volume of ionized air within the annular space of the chamber. The current to be measured may typically range from a few pico amperes for a small dose of radio-pharmaceutical, up to several tens of nano-amperes for a ‘bulk’ shipment.
A crucial attribute of a properly configured dose calibrator is that (for a given isotope) the measured current signal should be proportional only to the amount of activity present, i.e., depending only on the number of radioactive decays per second, and should not vary substantially because of small variations in sample volume or small changes in placement of the sample within the chamber.
Finally, the ion-chamber dose calibrator must be shielded so as to prevent false readings due to ambient background radiation that may be present in the lab and—more important—to protect the pharmacist or technician from exposure to penetrating radiation from the dose being measured. The shield 8 is typically configured as a cylinder surrounding the chamber (shown in cut away view.) The mass of shielding depends on the size of the chamber and on the particular radioactive materials being processed, and may require as much as a four inch thickness of lead, or equivalent dense material when operated in a busy, commercial-scale radio-pharmacy formulating doses for Positron Emission Tomography (PET). Doses must also be removed from their primary shield in which they are shipped and/or stored before making an ion chamber measurement, exposing personnel to unshielded radiation.